am I allowed to sacrifice a creature thats already on its way to the graveyard like because it was killed in combat
am I allowed to sacrifice a creature thats already on its way to the graveyard like because it was killed in combat
No, but you can sacrifice it just before it dies. There is a moment after blockers are declare but before damage is dealt where you get priority. Say you have a Viscera Seer in play, and your opponent attacks with a 3/3. You block with a 2/2. Then, before damage is dealt, you can sacrifice the 2/2 to the Seer to scry.
Fluttershy is best pony.